The present invention relates to a storage device in which a magneto-resistive (hereinafter referred to as MR) effect is employed. In particular, the present invention relates to a magneto-resistive device and a magneto-resistive effect type storage device, which have high sensitivity and high density.
A solid storage device using a MR film was proposed by L. J. Schwee, Proceedings of INTERMAG Conference IEEE Transactions on Magnetics Kyoto, (1972) pp. 405. Various types of MRAM (magnetic random access memory) including word lines as current lines for generating a magnetic field and sense lines using MR films for reading data have been proposed (A. V. Pohm et al., IEEE Transactions on Magnetics 28 (1992) pp. 2356.). Such memory devices use an NiFe film or the like exhibiting an anisotropic MR effect (AMR) having an MR change ratio of about 2%, and thus the level of an output needed to be improved.
It was found that an artificial lattice film formed of magnetic films exchange-coupled through a nonmagnetic film to each other shows a giant MR effect (GMR) (A. V Baibich et al., Physical Review Letter 61 (1988) pp. 2472), and an MRAM using a GMR film was proposed (K. T. Ranmuthu et al., IEEE Transactions on Magnetics 29 (1993) pp. 2593.). However, the GMR film formed of magnetic films antiferromagnetically exchange-coupled to each other exhibits a relatively large MR change ratio, but disadvantageously requires a larger magnetic field to be applied and thus requires a larger current for writing and reading information than an AMR film.
While the above-described GMR film is an exchange-coupling type, one exemplary type of non-coupling GMR film is a spin valve film. Examples of this non-coupling type GMR film are those using an antiferromagnetic film (B. Dieny et al. Journal of Magnetic Materials 93 (1991) pp. 101.) and those using a semi-hard magnetic film (H. Sakakima et al., Japanese Journal of Applied Physics 33 (1994) pp. L1668). These spin valve films require a magnetic field as small as that required by the AMR films and still exhibit a larger MR change ratio than the AMR film. This proposal relates to an MRAM of a spin valve type using an antiferromagnetic film or a hard magnetic film and indicates that this storage device performs a non-destructive read-out (NDRO) (Y. Irie et al., Japanese Journal of Applied Physics 34 (1995) pp. L415).
The nonmagnetic layer used for the above-described GMR films is a conductive film formed of Cu or the like. Tunneling GMR films (TMR: tunnel magneto-resistance) using an insulating film of Al2O3 or the like as the nonmagnetic film have actively been studied, and MRAMs using the TMR film also have been proposed. In particular, a TMR film, which has a relatively high impedance, is expected to provide a sufficiently large output.
In the case of arranging magneto-resistive devices and operating them as an MRAM, storage cells made up of magneto-resistive devices are selected by sorting out direct bit lines and word lines. Even if a TMR film having excellent device selectivity was used, there were paths that pass through unselected devices, which became equivalent to the state in which resistance was connected in parallel, so that MR of one device cannot be energized sufficiently as the output. Furthermore, in accordance with the increase of storage capacity, this problem causes the S/N ratio of the output to decrease.
To solve the conventional problems mentioned above, it is an object of the present invention to provide a magneto-resistive device and a magneto-resistive effect type storage device, which have improved selectivity and output signals.
To achieve the above object, the present invention provides a magneto-resistive device including a first resistive device and a second resistive device connected in series, wherein at least one selected from the first resistive device and the second resistive device is a magneto-resistive device.
Furthermore, the present invention provides a magneto-resistive effect type storage device including a first resistive device and a second resistive device connected in series, wherein at least one of the resistive devices selected from the first resistive device and the second resistive device is a magneto-resistive device, and the magneto-resistive device as a single storage device is arranged two-dimensionally or three-dimensionally in plurality.
In the following description, the terms xe2x80x9cdevicexe2x80x9d and xe2x80x9celementxe2x80x9d are used under the same concept and will be unified in the term xe2x80x9cdevicexe2x80x9d.
According to the present invention, an effective magneto-resistive effect type storage device capable of controlling the bias applied to the magneto-resistive device can be achieved, which has excellent selectivity of magnetic storage cells when arranged in lines for constructing an MRAM, and which suppresses the deterioration of the S/N ratio even if the densification of the storage capacity is promoted.